Anodizing of aluminum coated objects



Patented June 17, 1958 ANGDIZIN G OF ALUMINUM COATED OBJECTS Harry La Tour and 1) Cameron Perry, Middletown, Ohio, assignors to Arrnco Steel Corporation, Middletown, Ohio, a corporation of Uhio No Drawing. Application July 21, 1951 Serial No. 238,020

'7 Claims. (Cl. 204-28) The electrolytic anodizing of aluminum articles is a process of long standing in the art, the purpose being to provide a coating of oxide so as to improve corrosion resistance and abrasion resistance, as well as for certain other purposes such as the provision of a decorative surface.

In the commonly used anodizing processes, the work piece is made the anode in an electrolyte which may vary Widely in composition. Among the chemicals used for the electrolyte are sulfuric acid, chromic acid, oxalic acid, ammonium hydroxide and sodium silicate. The formation of the desired coating usually requires a treatment of from 5 to 30 minutes in duration. A fairly low current density is employed, ranging from 1 to 20 amperes per square foot, and the temperature is commonly not over 95 F.

As exemplary of the commonly used anodizing processes, a 5% solution of chromicacid, employed as an electrolyte at 95 F., with a potential of 40 volts and a current density of from 1 to 5 amperes per square foot wili give a suitable oxide coating in 30 minutes of. treatment time. As another example, an electrolyte consisting of a to water solution of sulfuric acid, employed with a potential of from 15 to volts, but at a current density of about 14 amperes per square foot, will, with a treatment time of minutes, give a satisfactory coating at a somewhat lower temperature, namely 70 F. More active electrolytes will give coatings in substantially lessened time intervals. An electrolyte which is a 2.5% solution of fluoboric acid, used at 85 F., at a potential of 15 to 30 volts and with a current density of 10 to 20 amperesper square foot will give a satisfactory coating in from 5 to 10 minutes.

Aluminum coated ferrous bodies have attained considerable importance in the art. Aluminum coated iron wire or strip of sheet width is currently being made in large quantity by a coating procedure of continuous character as taught in the Sendzimir Patent No. 2,110,893. It has been realized that anodized coatings on aluminum coated ferrous bodies would be desirable to improve corrosion resistance, i. e. to prevent or inhibit the formation of White and red rust, which are the corrosion products of the coating and the base metal respectively. It is further believed that a tightly adherent oxide coating such as can be formed on aluminum articles by anodizingis of value in certain applications requiring heat resistance.

Hitherto known processes for anodizing aluminum are not, however, satisfactory for use in the treatment of aluminum coated iron and steel. Rather heavy oxide coatings are produced at the expense of several thousandths of an inch of the aluminum coating. The ferrous bodies, wire and strip, to which this invention relates, are of iron or mild steel, as those terms are understood in the art, and are quite thinly coated with aluminum. By aluminum we mean not only pure aluminum but also alloys of aluminum with other metals in which aluminum is the major constituent and which have melting points within the hot dipping range. This includes not only the normal impurities in aluminum, but also alloying ingredients deliberately added, such for example as silicon or magnesium, the purpose of which is usually though not necessarily to control the extent or nature of the alloying of the aluminum with the iron base in the hot dipping process. Seldom do these coatings attain a thickness greater than about .llOZ in, and in many instances, especially where rolling or drawing is practiced on the coated arti cles, the thickness of the coating will be of the order of .001 in. or less per side.

The conventional processes of anodizing in the art dissolve the aluminum at the surface more rapidly than the electric current can offset the action by oxidation. Thus, they tend to remove the coating completely from aluminum coated iron and steel. The long, slow deposition normally used to produce thick and hard coatings cannot heemployed for this reason. Furthermore, the strongly acid electrolytes cause pitting of the aluminum surface, an action which is probably due to the iron content of the aluminum coating, irregularly distributed iron-aluminum alloy particles in the coating, and other irregularly distributed impurities. Yet again, if uncoated iron or steel edges are present, they and adjacent areas are preterentially attacked by the electrolyte.

The primary purpose of this invention is the provision of a procedure for anodizing aluminum coated iron and steel which will not have the above defects and will successfully produce upon such materials tightly adherent oxide coatings having improved corrosion resistance and other desirable properties.

It is an obiect of our invention to provide a procedure for anodizing aluminum coated iron and steel which will produce such coatings without removing the aluminum.

It is an object of our invention to provide procedures for anodizing aluminum coated iron and steel in wire or strip form, which procedures are well adapted for use in connection with continuous processes for aluminum coating such articles, and which can be operated as an adjunct of such coating processes and at equivalent speeds.

These and other objects of our invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by that procedure and treatment, of which we shall now disclose exemplary embodiments.

In the attainment of the objects of this invention, we have discovered that with certain electrolytes which, as distinguished from those of prior art processes, are neither strongly alkaline nor strongly acid, and operating at exceedingly high current densities, far beyond those hitherto employed in anodizing processes, and at substantially higher temperatures than those hitherto employed, We can so correlate the electrolytic oxidation with the attack on the aluminum itself that we can secure the required coatings without dissolving away excess aluminum, and produce an aluminum coated iron or steel article which is anodized and which, at the same time, retains: an adequate coating of aluminum.

One of the most important requirements is the regulation of the pH between 6 and 8, with a preference for a range of 6 to 7. The electrolyte must not be so acid as to attack excessively the uncoated edges of the ferrous base, not so alkaline as to dissolve too much aluminum from the surface. These requirements, it should be pointed out, are to be met under electrolytic conditions. Our solutions are able chemically to dissolve aluminum, if used as a pickle or the like, but their action in this respect is greatly retarded by the electric current.

The concentration of the electrolyte is not critical, and our preference is for quite dilute solutions. It appears that the more dilute the solution, the better the result,

although this is limited by the increasing resistance of the solution.

A high temperature is important to speed up the oxidation. A temperature greater than 160 F. is required in our Work, with a range of substantially 170 to 180 F. preferred. Temperatures up to the boiling point of the solution or electrolyte are operative. We have found that temperatures below about 160 F. either give a coating which is too thin or a reaction rate which is too slow.

It is important that the current densities be very high to produce a rapid anodization, for the same reason that requires the use of high temperatures. The current falls off rapidly as the coating forms, and hence the expression of current density in amperes per square foot is not fully satisfactory in an operation in which strip, for example, is continuously being drawn through an anodizing solution.

We operate at an initial current density of about 200 amperes per square foot of the sheet or strip or greater. In a continuous anodizing procedure in which aluminum coated strip or the like is drawn through the anodizing bath, we have found that approximately 166 to 250 amperes per inch of strip width gives a satisfactory current density.

In a continuous strip operation, the starting current densities are variable until a complete anodic film is formed; then the conditions become stabilized. It is this stabilized operating range we have had in mind in giving the last mentioned current density figure. An expression of current density in terms of strip width is more useful than an expression in terms of square footage of area since the resistance of any particular area will change as the coating is formed.

The voltage applied must be above the oxygen overvoltage potential of the solution used so that oxygen can be liberated. The voltage should approach or be in the cut-off voltage range. The cut-off voltage is that above which an increase in potential fails to increase the current proportionally.

The cathode or cathodes may be of any conductive material not attacked by the solutions. Without limitation, stainless steel or low alloy steel cathodes will be found entirely satisfactory.

If the article to be anodized is a flat strip or sheet, the cathodes should be shaped and spaced from the strip in such a manner as to encourage uniform current density distribution across the strip Width. As in plating, this means a closer approach of the cathodes to the anode (strip) at the center of the strip, and a greater separation of the cathodes from the anode at its edges. For articles of other shapes, cathodes can be designed so as to give effective current distribution in the manner commonly employed in the plating art and as will be readily understood by the electrolytic worker.

We are not limited to the use of any particular apparatus, nor to the continuous treatment of aluminum coated wire or strip. In our process, exposed edges of the ferrous base material, or the ferrous base where it is exposed by flaws or imperfections in the aluminum coating, are not preferentially attacked, and do not interfere with the formation of satisfactory oxide coatings on the aluminum coating itself. We may employ any suitable electrolytic tank for the anodizing of individual articles, and in the case of a continuous anodizing process, any of the known continuous electroplating tanks with suitable changes of cathodes and electrolyte will serve our purpose. We may, for example, use the continuous plating apparatus of Noble Eugene Hays, Patent No. 2,512,328 of June 30, 1950.

In an exemplary embodiment of our new process, we employ an electrolyte which has a pH value of about 6.5. Chromic acid is added to water in proportions to produce a 5% solution, and sodium hydroxide is added until the above mentioned pH is attained. The aluminum coated piece is made the anode in this electrolyte, there being a suitable cathode or cathodes of the type ordinarily employed in anodizing procedures. With the electrolyte at a temperature of about 175 F., a current density of about 166 amperes per inch of strip Width is employed at a potential of about 42 volts. This gives a satisfactory oxide coating in a very brief length of time, namely about 15 seconds. At the conclusion of the treatment, the piece or article is rinsed, scrubbed, and dried to avoid stains.

In another exemplary embodiment of our process, chromic acid is dissolved in Water to produce about a 3% solution, and potassium hydroxide is added to give a pH value of about 6.85. With the electrolyte at a temperature of substantially 170 F., a current density of substantially 250 amperes per inch of strip width at a potential of substantially 38 volts, a satisfactory oxide coating is produced in about 17 seconds.

We are not limited to the use of chromic acid as an electrolyte former. The electrolyte should, chemically, be a mixture of an acid and a base in the proper amounts so that the pH can be varied between the limits set forth above. The acid should be non-harmful to aluminum and aluminum alloys, and the combination of acid and base should produce a strongly oxidizing effect on the aluminum under the conditions described. By way of example, a mixture of acetic, sulphuric, nitric, phosphoric or citric acid and ammonium, sodium, magnesium or potassium hydroxide will produce satisfactory results. The base should be such that the positive ion of the electrolyte will not plate out on the strip being anodized. This is accomplished by using a base the positive ion of which is above the electrode potential of the aluminum or aluminum alloy coating on the strip. However, salts used for the electrolyte may contain metals which are not above aluminum in the electromotive series providing such metals are tied up as part of a negative ion. An example is the chromium in sodium chromate or sodium dichromate. If the positive ion of the electrolyte salt is below aluminum in the electromotive series, it will plate out on the aluminum coating. Thus, chromium could not be used as the positive ion in the electrolyte.

It has been found that carbonates and chlorides cannot be used since they cause pitting. Water containing dissolved carbon dioxide, such as untreated dc-ionized water, should be avoided.

In a continuous anodizing operation in accordance with our teachings, the speed of travel of the strip or wire should be fast enough to prevent excessive attack on the coating. This indicates, in a continuous treatment apparatus of feasible length, a lower limit of speed of about 3 to 5 feet per minute. There is, however, no upper limit on the speed since the treatment apparatus may be made as long as desired to give required time for treatment. We have operated at speeds up to and beyond 40 feet per minute and our experience has been that in general the faster the speed, the better the results. The time required for anodizing the aluminum strip in our process is of the order of 15 to 60 seconds, as compared to the 5 to 30 minutes or longer required by the conventional processes for anodizing aluminum.

Modifications may be made in our invention without departing from the spirit of it. Having thus described our process in certain exemplary embodiments, what we claim as new and desire to secure by Letters Patent is:

1. A process of anodizing iron or steel articles having an aluminum coating of substantially 0.001 to 0.002 inch thickness which comprises making them the anodes in a solution of chromic acid and sodium hydroxide having a pH of substantially 6 to 7, at a temperature of substantially F. for 15 seconds, at an initial current density of substantially 200 amperes per square foot, and a potential of substantially 42 volts.

2. A process of anodizing ferrous articles having an aluminum coating of substantially 0.001 to 0.002 inch thickness which comprises making said articles the anodes in an electrolyte having an oxidizing effect under electrolytic conditions, having a temperature of substantially 160 to 212 F. and employing an initial current density of at least substantially 200 amperes per square 7 foot and a time of treatment of substantially to 60 seconds, the said electrolyte comprising in water solution an acidic material chosen from a class consisting of acetic, sulfuric, nitric, phosphoric, citric and chromic acids and mixtures thereof, and a base chosen from a class consisting of ammonium, sodium, magnesium and potassium hydroxides and mixtures thereof, the said electrolyte having a pH substantially in the range of 6 to 8.

3. A process of continuously anodizing ferrous strandlike articles having an aluminum coating of substantially 0.001 to 0.002 inch thickness which comprises making said articles the anodes in an electrolyte which is a Water solution of reaction products formed by reacting an acidic material chosen from a class consisting of acetic, sulfuric, nitric, phosphoric, citric and chromic acids and mixtures thereof, and basic substances chosen from a class consisting of ammonium, sodium, magnesium and potassium hydroxides and mixtures thereof, said electrolyte having a pH substantially in the range of 6 to 8 and a temperature of substantially 160 to 212 F. and employing a current density represented by substantially 166 to 250 amperes per lineal inch of Width of the strand-like articles for a time of treatment of substantially 15 to 60 seconds, at a speed of travel of said strand-like articles of substantially 3 to 40 feet per minute.

4. A process of anodizing ferrous material in strand form having an aluminum coating of substantially 0.001 to 0.002 inch thickness, which comprises moving said material through an electrolyte at a speed of travel of substantially 3 to 40 feet per minute, said electrolyte having a temperature of substantially 160 to 212 F., said electrolyte being made by dissolving chromic acid in water and adding sodium hydroxide to give a pH between 6 and 7, electrolytically treating said material in said electrolyte as an anode, the current density employed being represented by substantially 166 to 250 amperes per lineal inch of width of the strand, the time of treatment being substantially 15 to 60 seconds.

5. The process as claimed in claim 4 wherein the potential is of the order of 42 volts.

6. A process of anodizing ferrous material in strand form having an aluminum coating of substantially 0.001 to 0.002 inch thickness, which comprises continuously moving said material through an electrolyte at a speed of travel of substantially 3 to feet per minute, said electrolyte having a temperature of substantially 160 to 212 F., said electrolyte being made by dissolving sulfuric acid in water and adding an alkali metal hydroxide to give a pH between substantially 6 and 7, electrolytically treating said material in said electrolyte as an anode, the current density employed being represented by substantially 166 to 250 amperes per lineal inch of width of the strand, the time of treatment being initially 15 to seconds.

'I. A process of anodizing ferrous material in strand form having an aluminum coating of substantially 0.001 to 0.002 inch thickness, which comprises continuously moving said material through an electrolyte at a speed or travel of substantially 3 to 40 feet per minute, said electrolyte having a temperature of substantially to 212 F., said electrolyte being made by dissolving chromic acid in water and adding potassium hydroxide to give a pH between 6 and 7, electrolytically treating said material in said electrolyte as an anode, the current density employed being represented by substantially 166 to 250 amperes per lineal inch of width of the strand, the time of treatment being substantially 15 to 60 seconds.

References Cited in the file of this patent UNITED STATES PATENTS 1,916,586 Robinson et al. July 4, 1933 2,262,967 Schenk Nov. 18, 1941 2,356,543 Sonnino et al. Aug. 22, 1944 FORElGN PATENTS 226,536 Great Britain Oct. 8, 1925 382,287 Great Britain Oct. 21, 1932 OTHER REFERENCES Gordon: Physical Review vol. 24, pp. 60-71, 1907. 

2. A PROCESS OF ANODIZING FERROUS ARTICLES HAVING AN ALUMINUM COATING OF SUBSTANTIALLY 0.001 TO 0.002 INCH THICKNESS WHICH COMPRISES MAKING SAID ARTICLES THE ANODES IN AN ELECTROLYTE HAVING AN OXIDIZING EFFECT UNDER ELECTROLYTIC CONDITIONS, HAVING A TEMPERATURE OF SUBSTANTIALLY 160* TO 212*F. AND EMPLOYING AN INITIAL CURRENT DENSITY OF AT LEAST SUBSTANTIALLY 200 AMPERES PER SQUARE FOOT AND A TIME OF TREATMENT OF SUBSTANTIALLY 15 TO 60 SECONDS, THE SAID ELECTROLYTE COMPRISING IN WATER SOLUTION AN ACIDIC MATERIAL CHOSEN FROM A CLASS CONSISTING OF ACETIC, SULFURIC, NITRIC, PHOSPHORIC, CITRIC AND CHROMIC ACIDS AND MIXTURES THEREOF, AND A BASE CHOSEN FROM A CLASS CONSISTING OF AMMONIUM, SODIUM, MAGNESIUM AND POTASSIUM HYDROXIDES AND MIXTURES THEREOF, THE SAID ELECTROLYTE HAVING A PH SUBSTANTIALLY IN THE RANGE OF 6 TO
 8. 